Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!crdgw1!uunet!pilchuck!dataio!fnx!nazgul!bright
From: bright@nazgul.UUCP (Walter Bright)
Newsgroups: comp.lang.c++
Subject: Re: Implementing inheritance
Keywords: C++ inheritance implementation description
Message-ID: <291@nazgul.UUCP>
Date: 28 Mar 91 19:34:04 GMT
References: <1991Mar14.233914.14777@evax.arl.utexas.edu> <1885@news.tcs.com> <3398@litchi.bbn.com> <626@taumet.com>
Reply-To: bright@nazgul.UUCP (Walter Bright)
Distribution: all
Organization: Zortech, Seattle
Lines: 27

In article <626@taumet.com> steve@taumet.com (Stephen Clamage) writes:
/Why would you need to know anything at
/all about virtual tables to learn C++?

Perhaps an example would help. I knew a guy in college once who was just
learning to program. He was very meticulous, and carefully read all the
pertinant manuals. Part of the program was used to write bytes to a file.
He modularized the function that did this, all to proper practice. It turns
out that the program ran incredibly slowly. He pulled his hair out trying
to figure out why, then at wit's end showed the program to his roommate.
His roommate found the problem immediately, it seems that in the function
to write the output to a file, in the inner loop he opened the file for
append, wrote a byte, and closed the file! Nothing anywhere in the manuals
said that this was the wrong way to write a file...

The point is, although theoretically you don't need to know how things are
implemented in order to use them, in the real world it is necessary to
understand what is going on. Not knowing how various C++ constructs are
implemented, and what the various runtime tradeoffs are, will inevitably
result in inefficient and large programs.

I recommend that professionals occaisonally look at the assembler output
of their compiler on their production code, the results can be very
enlightening! :-)

Disclaimer: I don't subscribe to the school of thought that program efficiency
is irrelevant. (It's only irrelevant for one-shot or rarely used programs.)